Deposition of gold films



United States Patent 01 lice DEPOSITION OF GOLD FILMS Donald J. Levy,Mountain View, Calif., assignor to Lockheed Aircraft Corporation, LosAngeles, Calif. No Drawing. Filed July 2, 1963, Ser. No. 292,433 Theportion of the term of the patent subsequent to June 2, 1986, has beendisclaimed Int. Cl. C23c 3/02 US. Cl. 117-35 11 Claims This inventionrelates to the deposition of gold films on metallic and nonmetallicsurfaces, and more particularly to a chemical reduction method forproducing said films, to solutions used in said method, and togold-plated articles produced by said method.

The deposition of gold films on glass and other nonmetallic substratesby a chemical reduction method involving generally the reduction of agold compound to metallic gold by a reducing agent has been described ina 1953 work by Samuel Wein entitled Gold Films (Metallic Coatings onNon-Metallic Materials, vol. and published by the US. Department ofCommerce, Ofiice of Technical Services. The rates of film deposition inmany of these methods have been relatively slow, requiring from fiveminutes to twenty-four hours of immersion in the plating solutions.Other methods involve relatively complicated process steps. Still othermethods have required elevated temperature heating of the substrate orof the solution. In another method involving the spraying of gold filmson glass, the exposed surface of the film, or the side not in contactwith the glass, has a dull, brownish appearance. The rate of golddeposition in this method is slow, and the dull appearance of theexposed gold surface prevents its use for other applications.

In contrast, the present invention provides for a relatively fast,simple, and economical method of depositing gold films on both metallicand nonmetallic surfaces. Only conventional apparatus is required, andthe method may be carried out at ambient temperatures. The filmsdeposited by the present method are continuous, adherent, and bright,making them suitable for many different applications in industry.

According to the present invention, gold films are produced on variousmetallic and nonmetallic surfaces by contacting said surfaces with asolution containing a gold salt and a gold complexer and anothersolution containing hydrazine as the reductant for the gold. Upon mixingof the two solutions, a film or bright metallic gold is deposited on thesurface. Many different surfaces may be coated by the method of thepresent invention. They include metals, glass, plastics, ceramics, andpainted surfaces. Metals such as silver, copper, and iron can be coateddirectly by the method of the present invention. Other metals, such asmagnesium and aluminum, which are too active electrochemically toreceive the gold film directly, may be coated by the present methodafter providing an anodic oxide or other suitable barrier coating, suchas lacquer.

Unlike autocatalytic chemical reduction processes, the present methoddoes not require a catalytic surface for the deposition of the gold. Allthat is required is that the surface be clean and free of hydrophobicsubstances so that wetting can occur. For the cleaning of glass andother surfaces, ordinary household cleansers and detergents have beenfound to be suitable. To increase adherence of the gold film, thesurfaces may be abraded, chemically etched with a suitable solvent, orotherwise treated to increase micro roughness or polarity.

More specifically, the present gold solution is prepared by dissolvingin water, a gold salt, such as gold chloride, gold bromide, sodium goldthiosulfate, or other soluble 3,515,571 Patented June 2, 1970 salt in aconcentration in excess of about 10- moles per liter. It has been foundthat the concentration of the free gold ions to be reduced, in the formof the aquocoordinated gold ion which is presumably Au(H O) should notexceed 10- gram ions per liter. The gold in excess of this concentrationis complexed by a suitable complexer or ligand. These ligands usuallycontain a nitrogen or oxygen donor atom and include alkali metalcarbonates, alkali metal hydroxides, ammonia, and aliphatic aminescontaining from 1 to about 8 carbon atoms and from 1 to about 5 nitrogenatoms. These complexers may be used singly or in combination with eachother. The pH of the gold metal solution may range from about 5 to about11, with a near neutral condition being preferred.

Table I below shows some examples of gOld metal solutions which can beused in the method of the present invention. All of these solutions maybe used for rapidly depositing a specular film onto glass to producehigh quality gold mirrors. In addition, the solutions indicated with anasterisk may be used to coat opaque articles. The coatings producedthereon are extremely bright and decorative, particularly when thesurface of the article is smooth.

TABLE I.GOLD SOLUTIONS Gold salt, moles The gold solutions containingethylenediamine are the preferred solutions since they are the moststable and may be stored for long periods of time without the formationof appreciable quantities of elemental gold or insoluble gold compounds.These solutions preferably contain from about 0.005 to about 0.2 moleper liter of a gold salt and ethylenediamine in a molar ratio of theethylenediamine to gold equal to about 2, or greater. The pH of thesesolutions is preferably maintained at from about 6 to about 8.

In preparing the gold solutions, it is important that the ingredients bemixed in the prescribed manner so as to avoid the formation of insolubleprecipitates. In accordance with the present invention, the gold salt,for example gold bromide, is first dissolved in water and thenneutralized with sodium hydroxide or some other soluble alkali metalhydroxide. The ligand, for example diethylenetriamine, is separatelydiluted with water and then neutralized with an acid such as hydrobromicacid. The gold salt solution and the ligand solution are then mixed toform a final'gold solution containing free and complexed gold ions inthe concentrations previously specified.

The present reductant solution may contain from about 0.1 to about 6moles per liter of hydrazine and from 0 to about 2. moles per liter ofan alkali metal hydroxide. For spray applications of the gold andreductant solutions, the reductant solution preferably contains from 2to 5 moles per liter of hydrazine and less than 1.5 moles per liter ofalkali metal hydroxide.

Adjuncts such as wetting agents, brighteners, and pH buffers may also beadded to either the gold metal solution or to the reductant solutionwithout departing from the scope of this invention.

In the preferred form of the present invention, the gold solution andthe reductant solution are sprayed simultaneously onto the surface ofthe article to be coated. The

spraying may be done at ambient temperatures and with spray equipment ofthe type commonly used for applying paints. These include airless sprayequipment or atomizers which employ air or other gases such asfluorocarbons and hydrocarbons. Preferably, the surface of the articleto be plated is initially wetted with the reductant solution before thetwo solutions are simultaneously sprayed on the surface. Pressures inthe order of 20 to 30 pounds per square inch may be used in pneumaticatomizing spray guns, and the sprays may be applied from a distance ofabout one foot from the article. As in conventional spraying methods, itis desirable that the present spraying operation be carried out in wellventilated areas.

The thickness of the present gold coatings increases with the flow rateand time of spray. However, essentially any desired coating thicknessmay be obtained. As With other chemical reactions, the deposition rateincreases with the concentration of the reactants and the temperature.The present films deposit at rates of microinches per minute or more atambient temperatures. Thus, opaque coatings can be sprayed in a fractionof one minute.

The efiiciency of the present spray method is relatively high. Forexample, despite the rapid flow of solutions over the substrate, it hasbeen found that over half of the dissolved gold that is sprayed on aglass surface becomes deposited. Thin films can therefore be appliedquite economically by this method without resorting to the usualindustrial practice of recovering unused gold residues. The spray methodalso lends itself Well to the coating of large parts, such as passivesatellites, as well as miniature parts.

In addition to the simultaneous spraying of the gold and reductantsolutions, the tWo solutions may also be flowed simultaneously over thearticle in such manner that mixing of the solutions takes place at thearticle surface. Anothermethod of applying the coating in accordancewith the present invention is to immerse the article in the goldsolution and then add the reductant solution, with stirring, to the goldsolution. The following example is illustrative of this latter method.

A neutral gold solution was prepared which contained 0.05 mole per literof gold chloride and 0.1 mole per liter of ethylenediamine. A plate ofclean borosilicate type glass was immersed therein. A reductant solutioncontaining 0.6 mole per liter of hydrazine and 0.03 mole per liter ofpotassium hydroxide was prepared and added with stirring to an equalquantity of the gold solution. Gold deposition on the glass startedwithin 15 seconds and was complete within several minutes at ambienttemperature (22 C.).

The exact nature of the present process is not clearly understood.Apparently, the two reactants meet at the substrate surface and formvery tiny gold crystals. These particles form on the substrate andadhere to it. The reaction conditions are such that additional goldparticles nucleate and form rather than continue the growth of crystalsinitially deposited. The newly formed particles continue to adhere toone another and to the substrate, indicating that surface energy mayplay a role in this reaction. In any event, the net result is anunexpectedly bright, highly reflective film of fine, crystalline gold.Even at 400,000 magnification, the film surface is extremely smooth.

Gold films deposited by the method of the present in vention have manyuseful properties. For example, the electrical resistivity of thepresent films was measured by the two-point and four-point probetechnique on films de posited on frosted glass squares by thesimultaneous spraying of a gold solution and reductant solution preparedas follows: 50 ml. of 0.2. molar gold chloride solu tion were added to700 ml. of water and a sodium hydroxide solution was added until theacidity had been neutralized. 1.5 ml. of ethylenediamine were added to50 ml. of water and then hydrochloric acid was added until thealkalinity had been neutralized. The first solution was added to thesecond solution and the resulting gold solution was thoroughly mixedafter being brought to one liter with water. The reductant solution wasprepared by mixing 50 m1. of one molar sodium hydroxide with ml. of 20molar hydrazine, and then diluting to 1 liter with water. The gold andreductant solutions were simultaneously sprayed on seven glass plates.The film thicknesses ranged from 1300 to 6200 Angstrom units based onweight measurements. The average resistivity of these coatings was 6.4microohm-cm. as meas ured by the four-point probe method. This can becom pared with the accepted value of 2.4 mieroohm-cm. for massive goldat 20 C. Since the experimental resistivity values are based on theapparent surface area, they would actually be somewhat less and incloser agreement with the accepted value if a correction is made for theroughness of the frosted glass. Thus, it appears that the electricalresistivity of the present gold films is close to the Value for bulkgold and that these films would be useful as conductors, counterelectrodes, and contacts.

The optical properties of the present gold films, or more specificallythe capacity to absorb and emit radiant energy of certain wave lengths,were measure on films of about 2000 Angstrom units thickness depositedby spraying on anodized coupons of 6061 aluminum alloy and HMZImagnesium alloy. The gold solutions and reductant solutions wereprepared in a similar manner as that described for the resistivitytests. Table II below shows the optical results obtained.

TABLE II.OPTICAL PROPERTIES OF GOLD COATING The value for solarabsorptivity, as, of 0.3 is normal for gold surfaces. However, roomtemperature emissivity, e varies with substrate surface preparation andthe nature of the deposit. (These films exhibited a matte appearancesince the substrate surface was no smooth.) ERT can be altered asdesired by such techniques as polishing the substrate surface or bymodifying the spray process to deposit gold with a black appearance.These prop erties indicate the suitability of the present films for usein the space industry for passive thermal control surfaces, solarreflectors, and the like.

An especially useful application of the method of the present inventionis the spraying of gold films on glass. It has been found that suchfilms have a bright appearance on both the exposed and reverse surfaces.The coatings are free of imperfections and discolorations and thereforeare suitable for high quality mirrors. Both the adhesion and abrasionresistance of the films deposited on glass can be considerably improvedby a heat treatment cycle. Table III shows the optimum time-temperaturerelationship for films deposited on microscope slides made of aborosilicate glass.

Table III.-Heat treatment for gold film on borosilicate glassTemperature, C.: Time, hrs. 100 8 or more After heat treatment, the goldfilms show no separation or lifting from the glass when apressure-sensitive adhesive tape is adhered to the film and pulled off.The gold films also resist the abrasion produced by rubbing a rubbereraser over the film.

Obviously, other modifications and variatons of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that the scope of the present invention is to be limited onlyby the appended claims.

I claim:

1. A method for applying a gold film to a surface which comprisessimultaneously spraying said surface with an aqueous gold solutioncontaining a gold salt in an amount greater than about moles per liter,free aquo-coordinated gold ions in an amountnot exceeding 10 gram ionsper liter, and a ligand for complexing the remainder of the gold ions insolution, and with an aqueous reductant solution containing from about0.1 to about 6 moles per liter of hydrazine and less than about 2 molesper liter of an alkali metal hydroxide, said solu-. tions reacting todeposit a film of gold on said surface.

2. A method according to claim 1, wherein the gold salt is selected fromthe group consisting of gold chloride, gold bromide, and sodium goldthiosulfate.

3. A method according to claim 1, wherein the gold complexer is selectedfrom the group consisting of an alkali metal carbonate, ammonia, analkali metal hydroxide, and an aliphatic amine containing from 1 toabout 8 carbon atoms and from 1 to about 5 nitrogen atoms.

4. A method according to claim 1, wherein the reductant solutioncontains from 2 to 5 moles per liter of hydrazine and less than 1.5moles per liter of an alkali metal hydroxide.

5. A method for applying a gold film to a surface which comprisessimultaneously spraying said surface with an aqueous gold solutioncontaining from about 0.005 to about 0.2 mole per liter of a gold saltand ethylenediamine in a molar ratio of ethylenediamine to gold equal toabout 2, and with an aqueous reductant solution containing from 2 to 5moles per liter of hydrazine and less than 1.5 moles per liter of analkali metal hydroxide.

6. A method according to claim 5, wherein the pH of the gold solution ismaintained at about 6 to about 8.

7. A method for applying a gold film to a surface which comprisescontacting said surface with an aqueous solution containing a gold saltin an amount greater than about 10 mole per liter, free aquo-coordinatedgold ions in an amount not exceeding 10- gram ions per liter, and aligand for complexing the remainder of the gold ions in solution, andwith an aqueous reductant solution containing from about 0.1 to about 6moles per liter of hydrazine and less than about 2 moles per liter ofalkali metal hydroxide, said solutions reacting to deposit a film ofgold on said surface.

8. A method for applying a gold film to glass which comprises cleaningsaid glass, simultaneously spraying the glass surface with an aqueousgold solution containing from about 0.005 to about 0.2 mole per liter ofa gold salt and ethylenediamine in a molar ratio of ethylenediamine to'gold equal to about 2, and with an aqueous reductant solutioncontraining from 2 to 5 moles per liter of hydrazine and less than 1.5moles per liter of an alkali metal hydroxide, said solutions reacting todeposit a continuous film of bright metallic gold on said surface, andthen heating the glass surface.

9. A method for applying a gold film to a surface which comprisessimultaneously spraying said surface with an aqueous go-ld solutioncontaining at least 0.001 mole per liter of a gold salt and an aliphaticamine containing from 1 to about 8 carbon atoms and from 1 to about 5nitrogen atoms, the molar ratio of said amine to said gold salt beingfrom 1 to 10,1 and with an aqueous solution containing hydrazine andless than 2 moles per liter of an alkali metal hydroxide, the molarratio of hydrazine to gold salt being from 50 to about 1000.

10. A method for applying a gold film to a surface which comprisesproviding an aqueous gold solution containing at least 0.001 mole perliter of a gold salt and an aliphatic amine containing from 1 to about 8carbon atoms and from 1 to about 5 nitrogen atoms, the molar ratio ofsaid amine to said gold salt being from 1 to 10, providing an aqueousreductant solution containing hydrazine and less than 2 moles per literof an alkali metal hydroxide, the molar ratio of hydrazine to gold saltbeing from 50 to about 1,000, mixing the gold and reductant solutions,and immediately contacting said surface with the mixture of gold andreductant solutions.

11. A method foriapplying a gold film to a surface which comprisesproviding an aqueous gold solution containing at least 0.001, mole perliter of a gold salt and an aliphatic amine containing from 1 to about 8carbon atoms and from 1 to about 5 nitrogen atoms, the molar ratio ofsaid amine to said gold salt being from 1 to 10, providing an aqueousreductant solution containing hydrazine and less than 2 moles per literof an alkali metal hydroxide, the molar-ratio of hydrazine to gold saltbeing from 50 to'about 1,000, and mixing the gold and reductantsolutions in the presence of the surface, whereby a gold film isdeposited on said surface.

References Cited UNITED STATES PATENTS 625,733 5/1899 Barnes 117-10552,756,497 7/1956 Gale 117130 2,915,406 12/1959 Rhoda et al. 1171303,032,436 5/1962 Gastin et al. 117130 3,235,473 2/1966 Le Duce 117-471,953,330 4/1934 Andres 117105 2,355,070 8/ 1944 Harford 20446 OTHERREFERENCES Wein, The Glass Industry, May 1959, pp. 244-246, 280, 281.

Wein, The Glass Industry, June 1959, pp. 304-306, 330.

Wein, HydrazineIts Use In Mirror Making, reprint from August 1955 issueof the Glass Industry, 6, pp. 117- 35.

RALPH S. KENDALL, Primary Examiner U.S. Cl. X.R.

1. A METHOD FOR APPLYING A GOLD FILM TO A SURFACE WHICH COMPRISESSIMULTANEOUSLY SPRAYING SAID SURFACE WITH AN AQUEOUS GOLD SOLUTIONCONTAINING A GOLD SALT IN AN AMOUNT GREATER THAN ABOUT 10-3 MOLES PERLITER, GREE AQUO-COORDINATED GOLD IONS IN AN AMOUNT NOT EXCEEDING 10-16GRAM IONS PER LITER, AND A LIGAND FOR COMPLEXING THE REMAINDER OF THEGOLD IONS IN SOLUTION, AND WITH AN AQUEOUS REDUCTANT SOLUTION CONTAININGFROM ABOUT 0.1 TO ABOUT 6 MOLES PER LITER OF HYDRAZINE AND LESS THANABOUT 2 MOLES PER LITER OF AN ALKALI METAL HYDROXIDE, SAID SOLUTIONSREACTING TO DEPOSIT A FILM OF GOLD ON SAID SURFACE.